1. Field of the Invention
The present invention relates to an OCB (Optically Compensated Bend)-type liquid crystal display device, and particularly relates to an OCB-type crystal display device with high serviceability, which inhibits disturbance of the orientation surface due to a circumferential electric field or irregularity of the orientation plane.
2. Background Art
Liquid crystal display devices have rapidly become prevalent because they can easily display a large volume of information, displacing CRT type display devices.
Conventionally, the twisted nematic mode (hereinafter, called TN mode) has been mainly used, in which, the liquid crystal molecules are inserted between the upper and lower plates and are twist oriented by rotation of the molecular axis direction (hereinafter, called a director) by 90 degrees in plan view, and images are displayed by rotating the director in the vertical direction with respect to the substrates by a vertical electric field.
However, this TN mode has the problem that the angle of visibility is narrow. Accordingly, the TN mode has the problem not only in that images are not visible from an angle, but also in that, when the display screen becomes large as a result of progress in manufacturing large size screen, the clearness of images becomes different in the center and at edges of the screen, which results in non-homogeneous visibility. Although a technique to expand the angle of visibility by addition of a phase compensating plate is disclosed in Japanese Unexamined Patent Application, First Publication No. Hei 6-75116, it is difficult to completely compensate for the twist structure of the TN mode liquid crystal, and there is still demand for a solution of this problem.
In order to solve the problem of the small angle of visibility, attention has been given to a system, in which a phase compensating plate is applied to each bend oriented cell. The system in which the phase compensating plate is combined with the bend oriented cell is called OCB (optically compensated bend, or optically compensated birefringence, and the quick response speed of OCB devices is attracting considerable attention. An explanation of OCB devices is as follows.
The OCB device has a structure in which bend oriented cells and phase compensating plates for compensating the phase of the liquid crystal layer are arranged between two substrates. There are various types of OCB devices that use various types of phase compensating plates. One type of OCB devices uses a phase compensating plate having a negative birefringence, the other device uses a biaxial phase compensating plate disclosed by Kuo in SID ""94 Digest, and still another device uses a pair of upper and lower phase compensating plates having negative birefringence with a hybrid arrangement, which is disclosed in Japanese Unexamined Patent Application, First Publication No. Hei 10-197862.
The liquid crystal cell used in the OCB devices is formed by inserting liquid crystal between two substrates, the opposing surfaces of which are orientation treated in one direction, and the liquid crystal Lc is oriented at the interface of the substrate surfaces with a tilt angle range (xcex8,xe2x88x92xcex8). When no electric field is applied to the liquid crystal cell in the above state, the tilt angle decreases to zero towards the center of the liquid cell. The tilt angle is reduced towards the center and the tile angle becomes zero at the center of the cell gap, where the liquid crystal molecules are oriented in parallel to both upper and lower substrates (this state is called a spray orientation). If the liquid crystal molecules are arranged in the above-described orientation, the desired wider angle of visibility is not obtainable. In the OCB mode, it is necessary for the liquid crystal by, for example, the application of a high voltage to the cell gap, to be rearranged in an arcuate structure (this state is called a bend arrangement), wherein the liquid crystal molecules at the center of the cell gap stand up and the tilt angle of the liquid crystal molecules at the interface with the substrate becomes zero.
In the OCB mode, the display is operated by controlling the tilt angle of liquid crystal molecules not located at the center of the cell gap, while the liquid crystal molecules at the center of the cell gap are maintained in the standing state. For example, when the phase compensating plate is arranged to conform with a dark representation of the display when the tilt angle of the liquid crystal molecules not located at the gap center is small (reclined), while the liquid crystal molecules at the gap center are standing as shown in FIG. 17A, the bright representation is obtained when the tilt angle of the liquid crystal molecules not located at the gap center is large (standing) while the liquid crystal molecules at the gap center are also standing as shown in FIG. 17B.
However, problems arise when the bend-type orientation is applied to active matrix liquid crystal display devices.
The first problem is that the liquid crystal molecules oriented in the bend-type orientation is liable to be affected by nearby electric fields parasitically generated between the pixel electrodes and wiring. For example, when a potential difference is generated between a pixel electrode on a particular pixel region and the signal line adjacent to the pixel electrode on a active matrix substrate, this potential difference generates an electric field in the liquid crystal layer parallel to the substrate, which causes the liquid crystal molecules in the region where the liquid crystal orientation is affected by the electric field to rotate and the liquid crystal in this region in the bend-type orientation is converted into a twist-type orientation. When the liquid crystal is converted to a twist orientation, standing of the liquid crystal molecules is suppressed and it becomes difficult to rearrange them in the bend orientation. Since the desired birefringence cannot be obtained in the area where the bend orientation is deformed, the image quality declines. When an orientation core is formed, even locally, in the pixel area in question, this orientation tendency propagates due to the liquid crystal characteristics, which also causes degradation of the image quality by disturbing the liquid crystal arrangement.
The other problem of the bend type orientation is that the liquid crystal molecules near the substrate are liable to be affected by irregularity of the substrate surface. In general, on the substrate surface, a number of irregular portions are present due to thin film transistors or protective insulating layers formed on the substrate by repeated lamination and etching. The liquid crystal layers tend to orient along the slopes of the irregularity, and the tilt angles of some liquid crystal molecules turn to the opposite direction to the normal orientation, which is called the reversed tilt phenomenon, and thus the bend orientation is disturbed.
As described above, in order to apply the bend type orientation mode to the active matrix liquid crystal display, disturbance of the bend type liquid crystal orientation caused by the nearby electric field or the irregularity of the substrate surface must be securely excluded.
It is therefore an object of the present invention to provide a liquid crystal display, in which the bend-type liquid crystal orientation mode is not disturbed by a horizontal electric field or irregularities of the substrate surface.
The present invention provides a liquid crystal display device defining a rectangular pixel region defined by a plurality of scanning lines arranged in parallel and a plurality of signal lines crossing the scanning lines, in which a liquid crystal is inserted between two facing substrates comprised of an active matrix substrate including pixel electrodes and thin film transistors and the other one is a transparent substrate including common electrodes, and the opposing surfaces of the active matrix substrate and the transparent substrate are orientation treated in the same direction, wherein the orientation direction is limited within xc2x145xc2x0 for the short side direction of the pixel region.
In the above structure, it is preferable for the signal lines to extend along the direction of the long side of the pixel region.
In general, the pixel region of a color liquid crystal display device is formed in a rectangular shape, and pixel regions having color filters of red, green, and blue are sequentially formed along the scanning lines in the short side of the pixel region. Thus, the signal lines extend along the long side direction of the pixel region. In this case, an nearby electric field is generated in between the surrounding wiring and the pixel electrodes. The size of the area where the nearby electric field is generated is apparently larger in the long side direction than that in the short side direction. Accordingly, the effect of the electric field on the liquid crystal molecules is larger along the long side direction than that along the short axis direction. In the liquid crystal display device of the present invention, since both substrates are orientation treated in the same direction and since the orientation direction is within xc2x145 degrees of the short side of the pixel region, when no voltage is applied between the pixel electrode and the common electrode, the liquid crystal molecules are oriented in the parallel or close to the parallel direction to the short side of the pixel region.
Under these conditions, when a potential difference is generated between the pixel region and the signal lines, the lines of force of the horizontal electric field generated by this potential difference points to the short side of the pixel region. Accordingly, the force line tends to be parallel or semi-parallel to the orientation direction of the liquid crystal molecules and generation of the twisted orientation core by the nearby electric field around the long axis of the pixel region can be suppressed. In the liquid crystal display device, since the directions of the short sides of all pixel regions are the same, and since the liquid crystal molecules are arranged within xc2x145 degrees to the short axis direction of the pixel, the liquid crystal molecules are oriented in parallel or close to parallel to the short axis of the pixel when no voltage is applied between the pixel electrode and the common electrode.
The present invention provides a liquid crystal display device, assembled by facing an active matrix substrate, which comprises a plurality of rectangular pixel regions, each of which is surrounded by one of a plurality of a scanning lines arranged in parallel and one of a plurality of signal lines crossing said plurality of scanning lines through an insulating layer and each of which comprises a pixel electrode and a thin film transistor, and a transparent substrate provided with a common electrode, inserting a liquid crystal therebetween, and the opposing surface of the active matrix substrate and the opposing surface of the transparent substrate are treated so as to have the same orientation direction, wherein the pixel electrode is formed in a layer located closer to the common electrode than the signal lines and the scanning lines.
The change from the spray orientation to the bend orientation is induced by the vertical electric field generated between the pixel electrode and the common electrode. At this time, when the distance between the signal lines and the common electrode is longer than the distance between the pixel electrode and the common electrode, the electric field generated between the surrounding wiring and the pixel electrode is hard to be effused into the liquid crystal layer, so that the bend-type orientation is hard to disturb. In order to arrange the pixel electrode at a closer position to the common electrode than the positions of the signal and scanning lines to the common electrode, an insulating layer is formed between the signal and scanning lines and the pixel electrode.
In the above structure, the end portions of the pixel electrode preferably overlap with the end portions of the signal and scanning lines through an insulating layer.
The electric field generated between the pixel electrode and the signal and 15 scanning lines is generated mainly from an origin located at respective end portions. Accordingly, if the end portions of the pixel electrode overlap through an insulating layer with the end portions of the signal and scanning lines, since the electric field generated between the pixel electrode and the signal and scanning lines is generated at the rear side of the pixel electrode, the effect of the vertical electric field is further reduced.
The present invention further provides a liquid crystal display device, assembled by opposing an active matrix substrate, which comprises a plurality of rectangular pixel regions, each of which is surrounded by one of a plurality of a scanning lines arranged in parallel and one of a plurality of signal lines crossing said plurality of scanning lines through an insulating layer and each of which comprises a pixel electrode and a thin film transistor, and a transparent substrate provided with a common electrode, inserting a liquid crystal therebetween, and the opposing surface of the active matrix substrate and the opposing surface of the transparent substrate are treated so as to have the same orientation directions, wherein a compensation electrode, which is capable of generating an electric field between said signal line or the scanning line, is formed in the same layer as that of the scanning line or the signal line between the scanning line and the signal line of said pixel.
In this liquid crystal display device, since the compensation electrode is formed in the same layer as the scanning line or the signal line, the force lines of the electric field generated from the scanning line or the signal line are absorbed by the compensation electrode, so that no effect is exerted on the liquid crystal layer and the orientation of the liquid crystal layer molecules is not disturbed. The compensation electrode is preferably formed so as to partially overlap with the pixel electrode. Thereby, the force lines of the electric field generated between the compensation electrode and the pixel electrode wrap around the rear side of the pixel electrode, which further reduces the effect on the liquid crystal molecules. Since it is preferable to maintain the potential of the compensation electrode equal to that of the common electrode, the compensation electrode is preferably connected to the common electrode or to the common wiring, maintained at the same potential as the common electrode.
The present invention further provides a liquid crystal display device, assembled by opposing an active matrix substrate, which comprises a plurality of rectangular pixel regions, each of which is surrounded by one of a plurality of a scanning lines arranged in parallel and one of a plurality of signal lines crossing said plurality of scanning lines through an insulating layer and each of which comprises a pixel electrode and a thin film transistor, and a transparent substrate provided with a common electrode, and the opposing surface of the active matrix substrate and the opposing surface of the transparent substrate are treated so as to have the same orientation directions, wherein the opposing surface of said active matrix substrate is formed into a flat surface.
The tilt angle of the liquid crystal molecules is liable to be affected by irregularities of the substrate surface. That is, if the substrate surface is inclined in the direction opposite to the tilt angle, the liquid crystal molecules will be oriented in undesirable directions, which may disturb the normal bend-type orientation of the liquid crystal molecules. If irregularities of the opposing surface of the active matrix substrate, formed by repeated lamination and etching, can be eliminated and a flat and smooth surface is obtained, tilting of the liquid crystal molecules in the undesirable direction can be prevented and the normal bend-type orientation can be obtained. The flat and smooth surface can be provided by formation of a thick organic insulating film.
The present invention provides a liquid crystal display device, assembled by opposing an active matrix substrate, which comprises a plurality of rectangular pixel regions, each of which is surrounded by one of a plurality of a scanning lines arranged in parallel and one of a plurality of signal lines crossing said plurality of scanning lines through an insulating layer and each of which comprises a pixel electrode and a thin film transistor, and a transparent substrate provided with a common electrode, and the opposing surface of the active matrix substrate and the opposing surface of the transparent substrate are treated so as to have the same orientation directions, wherein the opposing surface of the active matrix substrate and the opposing surface of the transparent substrate are inclined in the opposite direction to each other along the orientation direction.
The liquid crystal molecules in the vicinity of the substrate surface tilt following the inclination direction of the substrate surface. In the bend-type orientation, when in a non-voltage application mode, that is, when no voltage is applied to the liquid crystal, the liquid crystal molecules near the active matrix substrate and the liquid crystal molecules near the transparent substrate must be oriented into the opposite direction. If the opposing surfaces of both substrates incline in directions opposite to each other, the liquid crystal molecules near the opposing surfaces will tilt in the opposite directions along the oppositely inclined substrate surfaces and the liquid crystal molecules near both substrate surfaces form cores which induce the liquid crystal molecules to form the bend-type orientation.
The present invention also provides a liquid crystal display device, formed by facing an active matrix substrate, which comprises a plurality of rectangular pixel regions, each of which is surrounded by one of a plurality of a scanning lines arranged in parallel and one of a plurality of signal lines crossing said plurality of scanning lines through an insulating layer and each of which comprises a pixel electrode and a thin film transistor, and a transparent substrate provided with a common electrode, and the opposing surface of the active matrix substrate and the opposing surface of the transparent substrate are treated so as to have the same orientation directions, wherein, the opposing surface of the active matrix substrate and the opposing surface of the transparent substrate are inclined such that the gap width formed by both opposing surfaces along the orientation direction is V-shaped, which is wide at the center and narrow at both ends of the pixel region.
In this liquid crystal display device, since the active matrix substrate and the transparent substrate define the pixel region, each of these surfaces is formed such that the gap width is narrow at the end portion and wide at the center, so that the liquid crystal molecules tilt in the opposite directions at both end portions along the orientation direction. Thereby, the visual field angle dependence of the light shielding property of the display device decreases, which results in improving the visual field angle and the image contrast.